# Determining baseband S/N ratio and link margin for a satellite link

• L.J.Tibbs
In summary, to answer the question, we used Carson's rule to calculate the peak frequency of the signal, and then used the S/N formula for baseband FM to calculate the baseband SNR ratio for the voice channel. We then estimated the link margin by subtracting the FM threshold from the baseband SNR ratio.
L.J.Tibbs
Hi everyone,
I'm not sure if this is the right section to post this in, but the question doesn't really seem to fit any singular category, so I'll try this one (if someone has a better recommendation, please let me know!)

The question I'm seeking to answer is this:

A baseband signal of maximum frequency equal to 3.4 KHz is used in a SCPC-FM satellite link. A subjective improvement in baseband SNR of 7 dB is provided by de-emphasis. If the CNR of the receiver is 13 dB, calculate the baseband SNR ratio for the voice channel. If the FM demodulator has one FM threshold at 6 dB, estimate the link margin for this system. Take the RF channel bandwidth of the link as 45 KHz.

What I really need is a boost to get started. I'm confused by a few things. First, the 'subjective improvement of 7 dB' is something I have no idea what that means.

What I'm considering is using Carson's rule and then using the S/N formula for baseband FM.
(I apologize, this is my first time in the formum, so I'm still learning how to type formulas)
(S/N) = 3m^2*B/(2fm)*C/N*pw
where m = peak frequency/highest frequency present in modulation signal
B is what is determined from Carson's rule
pw is a subjective improvement factor
C/N is the carrier to noise ratio
and fm is the highest frequency present in the modulation signal

If I consider the 6 dB to be the carrier-to-noise ratio, I'd only need to determine the Carson's rule stuff and plug in. Am I right in thinking about it this way? Or am I way off base?

And the only other problem is that I'm not sure exactly how to plug in what I'm given to the formula, because I'm not sure if I'm given a peak frequency, highest frequency in the modulation signal, etc.

Can someone help to unconfuse me? Again, I'm not asking someone to do the problem for me, I just need a little help to get started.

Last edited:
Thanks in advance! First, let's look at the given information:- A baseband signal of maximum frequency equal to 3.4 KHz is used in a SCPC-FM satellite link. - A subjective improvement in baseband SNR of 7 dB is provided by de-emphasis.- The CNR of the receiver is 13 dB.- The FM demodulator has one FM threshold at 6 dB.- The RF channel bandwidth of the link is 45 KHz.Now, we can use this information to calculate the baseband SNR ratio for the voice channel. To do this, we will use Carson's rule, which states that the maximum frequency of the signal should be equal to half the bandwidth of the system (in this case, 45 KHz). So, we can calculate the peak frequency of the signal to be 22.5 KHz. We can then use the S/N formula for baseband FM to calculate the baseband SNR ratio: (S/N) = 3m^2*B/(2fm)*C/N*pwwhere m = peak frequency/highest frequency present in modulation signalB is what is determined from Carson's rulepw is a subjective improvement factorC/N is the carrier to noise ratioand fm is the highest frequency present in the modulation signalIn this case, our peak frequency (m) is 22.5 KHz, our highest frequency present in the modulation signal (fm) is 3.4 KHz, and our carrier to noise ratio (C/N) is 13 dB. We can then plug these values into the formula to calculate the baseband SNR ratio:(S/N) = 3*(22.5KHz/3.4KHz)^2*(45KHz/2*3.4KHz)*13dB/7dB= 8.28 dBFinally, we can estimate the link margin for this system. Since the FM demodulator has one FM threshold at 6 dB, the link margin is simply the difference between the baseband SNR ratio and the FM threshold, which is 8.28 dB - 6 dB = 2.28 dB. Therefore, the link margin for this system is 2.28 dB.

## 1. What is the purpose of determining baseband S/N ratio and link margin for a satellite link?

The process of determining baseband S/N ratio and link margin for a satellite link helps to evaluate the performance and reliability of the communication link. It also helps in optimizing the link design to ensure efficient use of resources and to maintain a stable and high-quality connection.

## 2. What factors affect the baseband S/N ratio and link margin?

The baseband S/N ratio and link margin are affected by various factors such as the transmit power of the satellite, the receiver sensitivity, the path loss, the system bandwidth, and the noise level in the communication channel.

## 3. How do you calculate the baseband S/N ratio?

The baseband S/N ratio is calculated by dividing the received signal power by the noise power. The received signal power is measured at the receiver, while the noise power is determined by considering the noise figure of the receiver and the noise temperature of the environment.

## 4. How is the link margin determined?

The link margin is determined by subtracting the required minimum S/N ratio from the actual baseband S/N ratio. The required minimum S/N ratio is determined based on the type of modulation used and the bit error rate (BER) required for the specific application.

## 5. What is the significance of a high link margin in a satellite link?

A high link margin indicates that the signal strength is significantly higher than the minimum required for the communication link. This provides a buffer against any potential signal degradation due to weather conditions or other external factors, ensuring a reliable and stable connection.

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